The present invention relates to an engine wall structure and to a method of producing an engine wall structure that comprises an inner wall, to which hot gas is admitted during engine operation, an outer wall, which is colder than the inner wall during engine operation, and at least two webs that connect the inner wall with the outer wall and delimit a cooling duct between said walls.
During engine operation, any cooling medium may flow through the ducts. However, in particular, the invention relates to engine wall structures and a process for manufacturing engine wall structures in which there is a plurality of such webs dividing the space between the walls into a plurality of ducts, in particular for cooling the firing chamber walls and the thrust nozzle walls of rocket engines driven with hydrogen as a fuel or a hydrocarbon, i.e. kerosene, wherein the fuel is introduced in the cold state into the wall structure, is delivered through the cooling ducts while absorbing heat via the inner wall, and is subsequently used to generate the thrust. Heat is transferred from the hot gases to the inner wall, further on to the fuel, from the fuel to the outer wall, and, finally, from the outer wall to any medium surrounding it. Heat is also transported away by the coolant media as the coolant temperature increases by the cooling. The hot gases may comprise a flame generated by combustion of gases and/or fuel.
Accordingly, the engine wall structure is preferably a thrust nozzle wall, preferably of a rocket engine.
According to prior art, the engine wall structures of regeneratively cooled combustion chambers for liquid propellant rocket engines, cooling channels or ducts are machined, for example by milling, in a sheet or core that will form the inner wall, or at least part of an inner wall. In the case of regenerative cooling, this inner wall sheet may mainly comprise copper or a copper alloy. However, other materials such as steel may also be used as the core. The resulting ducts are delimited by remaining webs, and may subsequently be filled with a filler material such as a conductive resin.
Subsequently, an outer cover, defining the outer wall, is applied to and attached to the projecting webs, for example by means of electro-deposition. The outer wall may comprise plural layers of a material such as nickel or a nickel-alloy. The outer cover may, possibly, also be attached to the inside of the inner wall sheet, thereby fully sur-rounding the core. The filler material, transformed by means of heating into a liquid state, is then drained off through an end of the respective duct.
However, prior art results in an insufficient control of the exact thickness of the remaining inner wall, due to the inherent problem of obtaining an exact milling depth in the inner wall sheet. As a result, the control of the heat transfer becomes less predictable than it would have been if the exact inner wall thickness had been known. Also the area of the cross section of the ducts depends of the milling depth. Since alterations of that area will result in correspondingly altered flow conditions in the duct, this will also affect the effective heat transfer and the possibility of predicting the latter.
Moreover, the requirements on the thermal conductivity of the inner wall and the webs may differ substantially. By regenerative cooling of an engine wall structure, by which the cooling medium has a high heat absorption capacity by the large coolant mass flow and largely comprises fuel to be used in a subsequent combustion process, the conductivity of the inner wall is much more decisive for the outcome of the cooling than is the conductivity of the webs. By so called dump cooling, by which the cooling medium has a low heat absorption capacity by a low coolant mass flow, the heat conductivity of the webs may be more decisive for the outcome of the cooling than will the conductivity of the inner wall. This insight has not been mentioned at all by prior art.
It is desirable to provide an engine wall structure and a method of producing an engine wall structure as initially defined, by which heat is effectively and predictably transferred from the inner wall to the outer wall through a cooling medium, preferably a fuel, in one or more ducts and through the material of the webs that delimit said duct or ducts and that connect the inner and outer walls.
The invention shall also present an engine wall structure the construction of which is such that it promotes the obtaining of a very precisely controlled inner wall thickness upon generation of the webs as well as a facilitated subsequent attachment of the outer wall to the webs, especially when the inner wall material is different from the outer wall material and not easily connected by any metal fusion process. The design of the engine wall structure should also be such that it takes into consideration the different heat conductivity requirements of the inner wall and the webs.
According to an aspect of the present invention, the webs are formed by application of a first material onto the inner wall, said inner wall being comprised by a second material of other composition and other heat conductivity than said first material.
Any suitable technique for applying the webs to the inner wall may be used, such as welding of solid pieces of the first material onto the inner wall. However, deposition of the first material, preferably electro-deposition, is preferred.
By building the webs by means of application thereof onto the inner wall, preferably by deposition and most preferably by means of electro-deposition, the thickness of that wall will not be affected like when the webs are produced through machining of the inner wall, while, at the same time, the height of the web can be very finely adjusted, for example by means of a final milling of the web top. By using materials of different composition and heat conductivity, the webs may be tailored for their individual, specific functions, especially regarding the conductivity. Subsequent to the formation of the webs, the outer wall is attached to the webs.
Preferably, a removable mask is placed onto said inner wall before the deposition of the webs is begun, said mask defining spaces in which the webs are deposited onto the inner wall. Thereby, a precise deposition of the web material is promoted.
According to a preferred embodiment the outer wall is connected to the webs by means of a metal fusion operation, preferably welding, and most preferably laser welding. Accordingly, the outer wall comprises a sheet or the like that is connected to the webs.
Preferably, the composition of the material of the webs is substantially equal to the composition of the material of the outer wall. Thereby, any metal fusion process for attaching the second wall to the webs is facilitated.
Preferably, the material of the inner wall has higher heat conductivity than the material used for the webs. This is typically an advantage in those cases when there is a regenerative cooling with a high coolant flow rate or when the cooling medium has a high density, such as when in liquid state, resulting in a high heat absorption, but still a relatively low temperature of the cooling medium and, accordingly, in a relatively low temperature of the webs and the outer wall. The heat conductivity of the material of the inner wall will be decisive for the amount of heat that will be transferred to the cooling medium. The webs and the outer wall may then, preferably, be made of a material of higher mechanical strength than the material of the inner wall, while their conductivity is of less importance. Preferably, regenerative cooling is applied to stage combustion cycle rocket engine nozzles or expander cycle rocket engine nozzles.
In a preferred embodiment, with rapidly flowing cooling medium or a cooling medium of high density, preferably liquid fuel, the inner wall comprises a copper or a copper-based alloy, and the webs comprise steel. Typically, this is preferred for a so-called regenerative cooling when hydrogen or kerosene to be used as fuel is also used as the cooling medium. The flow of the cooling medium should be such that a temperature well below the melting point of copper or copper alloy is obtained in the inner wall, preferably below 800 K. The use of a material with a remarkably lower heat conductivity, such as steel, for the inner wall, would result in a build up of a too high temperature in the inner wall and, as a result, a deterioration of the inner wall material.
Several materials, such as steel, used for inner walls and webs have relatively low heat conductivity at low temperatures. A low temperature of the cooling medium, for instance at the cooling duct inlet, will result in a low temperature of the engine wall webs, and a low heat conductivity thereof. Also, if the heat transferability of the cooling medium is poor, for example due to a low flow rate or due to a low cooling medium density, it would be desired to compensate this by the use of a highly heat conductive material, such as aluminum, for the webs, and possibly also for the outer wall. Therefore, according to one aspect of the invention, the material of the webs has higher heat conductivity than the material of the inner wall. This feature is preferred for so called dump cooling. Preferably, dump cooling is applied to gas generator cycle rocket engine nozzles.
If the cooling ability of the engine wall structure, including the cooling medium, is poor due to a low cooling medium flow rate or a low cooling medium density, the temperature of the inner wall might be to high for permitting the use of a highly heat-conducting material such as aluminum for the inner wall. In such cases it is preferred that the temperature resistance of the material of the inner wall is better than that of the web material. Thus, according to a preferred embodiment of the invention, the inner wall comprises steel or copper and the webs comprise aluminum or an aluminum-based alloy.
According to another aspect of the present invention, an engine wall structure comprises an inner wall, to which hot gas is admitted during engine operation, an outer wall, which is colder than the inner wall during engine operation, and at least two webs that connect the inner wall with the outer wall and delimit a cooling duct between said walls, characterised in that the webs are mainly comprised by a first material and that the inner wall is mainly comprised by a second material of other composition and other heat conductivity than said first material. Preferred embodiments of the engine wall structure of the invention include those embodiments that have been described above with regard to the inventive method, especially with regard to the specific compositions of the first and second materials.